Known techniques for detecting and quantifying chemical or biological molecules, and in particular nucleic acids, oligonucleotides, proteins, polypeptide sequences, or fragments of such compounds, make use of biochip type devices comprising substrates that are generally made of glass, silica, metal, or nylon, and having fixed thereon specific ligands (“probes”) for the molecules that are to be analyzed. The molecules in a sample that is brought into the presence of these ligands couple with those ligands that match them. Radiation that is substantially monochromatic is used to excite chromophore elements that are associated with the molecules, and these elements respond by emitting luminescence or phosphorescence at some other wavelength, thus making it possible to identify the molecules that are coupled to the probes.
The chromophore elements are formed by the molecules themselves or else they are fixed or grafted to the molecules, or indeed they may be formed by the substrate itself on which the molecules become grafted, and they have light-emitting properties that depend on the presence and the nature of the grafted molecules.
In known devices, the probes are fixed in an array on the substrates and they are situated at known locations. The light signals emitted by the chromophore elements serve firstly to locate the probes to which molecules have coupled and to quantify the quantities of molecules that have coupled with the probes. Nevertheless, it is necessary to have a relatively large number (104) of molecules in the same category in order to obtain a usable light signal. In addition, the trend is to place ever increasing numbers of probes on substrates of very small dimensions, so that it becomes very difficult to locate with sufficient accuracy, and thus identify, the probes that are carrying the chromophore elements from which the detected light signals originate.
The conventional solutions usually implemented for solving this problem consist in increasing the signal level exciting the chromophore elements in order to increase the response light therefrom, or else in increasing the quantity of molecules that couple and/or the quantity of probes, however those means are not very satisfactory for reasons of poor energy efficiency and cost. Increasing the intensity of light excitation runs the risk of photodegradation of the molecules, and increasing the quantities of coupled molecules and probes requires an increase in the quantities of reagents used, which reagents are expensive.
Other known means use optical detection systems of very wide numerical aperture and/or immersed systems (in order to increase refractive index) but they are not very compatible with requirements for rapid scanning of arrays of probes on substrates.
Proposals have also been made to associate such biochip type devices with various means for picking up a larger fraction of the light emitted by the chromophore elements in response to light excitation, and for concentrating the picked-up light on detection and measurement means.